Rotel Integrated Amplifier RA-931 Mk II
It's an amp; it's an integrated; yes, it's a Rotel!
And size doesn't matter, believe me
Product: Rotel Integrated Amplifier RA-931 Mk.II
Serial No.: 039-0221428
Manufacturer: Rotel - Japan
Price: app. $ 220 - $ 250 Euro
Reviewer: Dejan V. Veselinovic
Reviewed: April 2001
40 years down the road, Rotel has acquired an enviable reputation throughout the world. Over the years, from a me-too brand, they have evolved into an authentic approach company, with a general outlook and many awards for it in their collection. Yet for all that, what I have always liked about them, they were never boisterous and never made wild claims in any of their ads.
Rotel made their name in the budget sector. Most notably, they were among the first to follow the lead of the now legendary NAD budget amplifier, and to this day, Rotel is one of the most important budget sector market leaders. The model under review here, RA-931, is a model which has been around for a long time, and is currently in its Mk.II form.
On the back, left to right, are the RCA Cinch input and tape feed plugs, two sets of binding posts for two pairs of speakers and the captive power chord. There's a sticker which says "Made by ROTEL in China". Again, the markings are very straightforward, but also very legible. The photo doesn't show this, the plug pairs for phono and CD are of the same commercial quality as the rest, but are gold plated. A nice touch.
Inside, you'll notice the toroidal transformer first - somehow, it demand attention, as it is indeed a rare sight in this price class. Although it has no specific markings, off hand I'd rate it at 120-150VA, but I could be wrong. Now, I'm always concerned about toroid fixings, I've seen far too many almost wobble when yanked. So I yanked this one, but ultimately, I started shaking the table the amp was on. Bolted tight, real tight, this one - kudos to Rotel.
Next, you'll notice lots and lots of empty printed circuit board space. Near the input, you'll find the phono RIAA section. It's a straightforward affair, based on a Philips NE5532 op amp, which is surrounded by 1% metal film resistors and what appear to be good quality styroflex capacitors.
Some jumpers and centimeters away is the preamp line stage, which is based around a Burr-Brown OPA 2604 op amp. Again, quality 1% metal film resistors are in evidence. The same in the power amp sections, which are fully discrete, each using 11 transistors. The output stages use complementary pairs of SB817/SD1047 power transistors (100W, 160V, 12A, 10 MHZ); this is unusually generous for this price class, where most are happy using 80W devices. Blissfully, there are no output inductors, which means a more even damping factor across the audible band. Protection is by fuses only, one in each output line, rated at 4A.
The heat sink, while of good overall quality, is a little too small for my taste; if you're into higher power outputs, I don't think they could take too much for too long, so do be careful.
Inside, you'll find precious little wiring, and what little there is, is truly unavoidable. It boils down to connecting the power transformer, supplying the front plate power on indicator LED with power and, believe it or not, that's it. Excellent work, this.
Evidence of low price could be found elsewhere. For example, the switches and pots are of commercial quality, which is a nice way of saying they're cheap and nasty. Workmanship is typically Chinese - rough and ready, components are almost thrown in and soldered (however, the actual soldering is quite good). The filter section consists of two Rubycon 6,800uF/50V caps, enough, but just barely, I would have preferred larger values there.
Overall, this is a well designed unit, using above the norm components, constructed in a typical Chinese labor manner, with a well finished outward look.
As is well documented, I don't put too much stock in measurements. However, it remains a fact that they are not merely useful, but indispensable in designing, and useful in reviewing. So, my usual set of measurements was made.
Left channel DC offset was measured at 6 and 6.2 mV, which is bot commendably low and commendably similar - it is, in fact, practically the same thing.
Bias current showed a slight imbalance for the two channels, one being 9.5% above the absolute value of the other, which may or may not be significant. However, transistor matching is outstanding for both channel, as my 0.1% precision metering showed identical values for NPN and PNP devices on both channels. The actual imbalance could be set right using the only available trim pots on the board, these being of the usual junk quality. But they are also small, so changing them for multiturn, sealed precision types should be extremely easy.
Yet the feeling of higher than usual resolution never left me, no matter what I put on. This amp seems to be endowed with a capability to present what's on the CD in a more discriminating way than I would normally expect at this price. Female voices and larger choirs seem to especially benefit from this - female voices which can often seem slightly shrill here came out very well indeed, with no trace of shrillness. James Taylor, who can sound a little nasal elsewhere was in apparently very good health with the Rotel. While perhaps not stunning, these are in fact very good signs.
Nevertheless, at low levels of listening, the Rotel will deliver a little less bass than is normally expected; what it does deliver is good, but there's a bit lacking there.
This relative shortcoming will disappear when you turn it up even a bit more than quiet, let alone louder than usual (in listening room terms). Mark Knopfler's guitar will sound very much alive and kicking, Herb Alpert's trumpet will sound like it's made of brass not plastics and Manitas del Plata will come alive in the best sense of the word.
In absolute terms, this amp sounds more powerful than its specifications would lead you to believe. It had no regrets driving my not-so-gentle AR94 speakers to even very loud volume levels, seemingly without running out steam. In other words, the Rotel can - and does! - kick when required. Bear in mind here that the ARs are not only not so easy to drive, but are also less efficient than modern speakers, and by a nice 3 dB margin (meaning you need twice the power for the same loudness level).
Perhaps more impressive than the sheer volume it can produce is Rotel's unwavering control of the speakers. It grabs them and holds them, not as well as say Karan's IA-i1 (whose damping factor is 11 times greater), but as well as any in its class, and most in the class or two above it.
Since I was given a sample which will eventually end in somebody's home, I didn't try pushing it until it blew its fuse. What I did get below that level was too loud for usual listening anyway, even on the ARs, let alone the 4 dB more efficient JBL Ti600 speakers. B&M Acoustics AP17 is what this Rotel will more probably drive in real life anyway, and drive them it will, if necessary so loud that you'll be in trouble with the wife and/or neighbors long before the Rotel is in any real danger.
I also tried this amp with my standard AR94 as the load, my JBL Ti600 and B&M Acoustics AP17, a guest on test. In all cases, the Rotel showed no stress at any time under what could be called normal listening levels, which includes some powerful music. Naturally, it will benefit from more efficient speakers, just as any amp will, no matter what its power rating, but it obviously does not shy away from harder loads.
The change of test cables brought about the anticipated and well documented differences. Jamo's OFC speaker cable gave decent performance, but as usual, managed to loose some ambience, while transferring tons of power. Van den Hul's CS122 cable gave its usual neutral results, but restored the lost ambience and let the Rotel do its thing. The somewhat warm sounding Van den Hul 352 Hybrid cable, with its 5.5 mm cross section, transferred all the power of Karan with no qualms, let alone of the humble Rotel; in fact, I preferred this cable precisely for its warmth, and feel it to be the ideal partner for the Rotel.
My usual anti-vibration test included inserting SoundCare spikes underneath; this brought about some minor, but still audible, improvement. The overall clarity level increased somewhat, from top to bottom. I'd say the improvement level was worth the price, but as usual, the decision is up to you.
On the minus side, I didn't like the turn on/turn off thump at all; in this day and age, it's simply not necessary, and shouldn't be there. I didn't like the speaker binding posts either; yes, they will let you use banana plugs , but I would have preferred to have seen decent posts which will accept inserted, not wound, 5 mm speaker cables. And the power chord could have been of better quality. The heat sink is smaller than I like to see, and will not take larger powers for longer periods of time. Finally, the sound is a little thinner than I would have liked.
On the plus side, we have - er - everything else. Toroidal transformers appear as frequently as wisdom teeth in this class, but Rotel has one inside, of good quality and well affixed. The general concept is very much a purist approach, also very rare in this price class. The overall quality of semiconductors used is say two to three cuts above the norm for this price class - my much more expensive H/K 680 cannot boast that many 1% metal film resistors. The electrical execution, while somewhat rough in implementation, is nevertheless very well conceived, allowing for zero wiring, a very much audiophile concept. Two critical inputs, phono and CD, use gold plated jacks, a rare sight, but a well judged one.
On balance, we have here an audiophile concept on the cheap, where the word "cheap" refers to the cost of acquiring. The money went where it can sound its best, a VERY rare thing these days, and cost cutting was done where it hurts the least. Obviously, Rotel knows what it's all about, and they are sharing it with us in this (and presumably other) model.
So, if you are an audiophile with a shallow pocket, don't despair, there's somebody out there thinking about you, and he's called Rotel. Model RA-931 Mk.II proves this amply. If I was in that market segment looking for an integrated amplifier, I'd look no further than Rotel's RA-931 Mk.II - it's a top buy as far as I'm concerned.
Tweakers, here's your millennium chance! It's a simple unit, and changing the two op amps could produce some interesting results. Just to start you off, you could try OP275 (Analog Devices, PMI) in the phono section, it is about 50% faster than NE5532 and has a FET front end, and so should be less susceptible to cartridge aberrations than the NE. It's nominal noise level is about the same as that of the NE. For lower noise, you could try OP237 (Analog Devices, PMI) or LT1115 (Linear Technology) for ultra low noise, and there are other options.
The line level op amp could also stand some change; consider OP275 and AD826 (Analog Devices) as two possibilities, but Burr-Brown has some other nice op amps as well, like OPA627 (a bit expensive, though).
Then you could change those two capacitors for some larger capacity and better quality ones; about 10,000uF/50V should be enough.
Binding posts next - get some decent ones.
The rest is all yours.
© Copyright 2001 Dejan Veselinovic - https://www.tnt-audio.com
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